3 Answers
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Nitey Gavriel (Hilchos Sukkah pg. 249) writes that the common custom is not to make "Leshev Basuka" on Havdala as it is not considered "settled" (and cites Shevet Halevi 6:42 as a source). However he notes that some do, and in the footnotes points to the Chabad custom based on the Friediker Rebbe who mantains that Havdala is considered "settled". He writes that this was also the custom of the Chazon Ish and R' Moshe Feinstein and that some are careful to remain in the sukkah for a bit in order to avert the problem.

If Havdala is not considered 'settled' then how can we motzi each other? Also, how does remaining in the Sukkah a bit help?
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Double AA♦Oct 7 '12 at 1:05

@DoubleAA Why does Havdala need to be "settled" in order to be motzi someone else?
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MichoelOct 7 '12 at 1:07

See Tosfot brachot 43a sv ho'il. Also Beit Yosef 296 brings more discussion and achronim on ShA 296:6. Note that even according to those who stand they achieve keviut in a different way, but there is still keviut.
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Double AA♦Oct 7 '12 at 1:12

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@DoubleAA Good question. The Shevet Halevi quoted deals with it (and says that that's probably the reasons others do make le'shev basuka), and explains that since you can drink less than a revi'is of wine it is not considered "kvious" as far a sukkah is considered, although it is considered "kvious" with regards to Havdala.
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MichoelOct 7 '12 at 1:24

According to most Rishonim (including Rambam, Rif, Rosh and Tur (see Beit Yosef OC 639)) you say a Bracha whenever you walk into the Sukkah and this would be no exception. Some Achronim rule this way, led by the Gra (Beiur Hagra to OC 639:8).

For those who hold of the other opinion (Rabbein Tam: to say it on eating and thereby exempt the other instances of dwelling) then based on Mishna Berura OC 639:48 it seems that if you are not planning on eating grain products while in the Sukkah this time (ie before the next time you leave it for an extended period of time) then you should say a Bracha on the drink. If you are planning on eating a grain product before the next time you leave, then wait and say the Bracha then.